A metallic sheet such as zinc, iron and aluminium plates is usually applied with a coating from the esthetic and other functional points of view and often subjected to a pretreatment to improve adhesion of the coating and anti-corrosive property.
As the aforesaid pretreatment, there are such methods as phosphate treatment, chromate treatment, anodizing method and the like, and among them, particular attention is directed to the chromate treatment because a highly anti-corrosive coating can be obtained therewith.
Various proposals on this chromate pretreatment technique have been heretofore made for further improving the anti-corrosive property and adhesion of the coating.
These proposals include, for example, a technique for forming insoluble coating by controlling the ratio of hexavalent chromium to the total chromium in the chromate solution (e.g. Japanese Patent Publication No. 18217/64, ibid 37567/79, a technique for using, as binder, colloidal silica or water dispersible silica, in a coating type chromate composition to improve chromium pick-up (e.g. Japanese patent application Kokai No. 10834/77, ibid 92339/78), a technique for using a water soluble or dispersible resin for the same purpose (e.g. Japanese patent application Kokai 30235/77, ibid 49029/78), a technique for adding phosphoric acid, fluorine salt or the like as reaction promotor (e.g. Japanese Patent Publication No. 26525/69, ibid 42135/77), a technique for adding salts of such metals as Ni, Co, Mn, Zn, Ba and the like as film modifier (e.g. Japanese patent application Kokai No. 105486/81, ibid 34178/83) or a technique wherein the metallic substrate is first treated with a chomate solution containing a reaction promotor and then with a coating type chromate liquid.
However, in these methods, there is indeed an increase in initial corrosion resistance, but none of the methods can solve the essential problems inherently possessed by the chromate coating, of degradation in corrosion resistance caused by thermal change or degradation in corrosion resistance with time due to the reaction between active chromium in chromate coating and other constituents thereof.
On the other hand, compounding of anti-corrosive pigment such as solid chromate pigment, lead pigment and the like having comparatively lower solubility, to a coating composition for which a higher corrosion resistance is desired has been practiced under certain circumstances. However, there is a problem that chromium, lead and the like are liable to be dissolved out of the coating and hence the corrosion resistance be decreased with time.
Moreover, though it is not so serious as compared with the case of using water soluble chromium compound, such chromate pigment may react with other constituents of certain paints, and thus there are coatings and hence compositions which cannot include such chromate pigment. Even if a coating composition can stand such pigment, there exists a limit in the employable chromate pigments and in some cases, it will be essential to use a considerably larger quantity of pigment, and these facts will bring about additional, complicated and difficult problems in the combination of pigment and coating composition.
Under the circumstances, it would be an immeasurable advantage if a technique is established wherein a metal surface is effectively protected by a coating which is excellent in thermal resistance and is capable of fixing chromium compound on a metal surface that can never react with the coating resin and can exhibit excellent corrosion resistance for a longer duration of time, and on which any type of coating composition may be freely applied. The inventors, having endeavored to find a good solution to this problem, have found an ideal, metal coating pre-treating composition comprising a hexa-valent chromium containing compound having a specified range of solubility, a specific type of binder and an optional water soluble chromium compound. The invention has been made on the basis of this finding.